The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus. More particularly, the present invention relates to an image transferring device including an endless belt passed over a plurality of rotary bodies for conveying a recording medium and a cleaning member for cleaning the surface of the belt in contact therewith and transferring a toner image from an image carrier to a recording medium carried on the belt.
Generally, a copier, printer or similar image forming apparatus includes an image transferring device for transferring a toner image formed on an image carrier to a paper sheet or similar recording medium. One of conventional image transferring devices includes an endless belt passed over support rollers, or rotary bodies, and facing a photoconductive element or image carrier in an image transfer region. The belt conveys a paper sheet or similar recording medium to the image transfer region. At this instant, an electric field formed in the image transfer region transfers a toner image formed on the photoconductive element to the recording medium.
In the above-described convention image transferring device, the belt and photoconductive element contact each other with the intermediary of the paper sheet. It is therefore likely that toner deposited on the background of the photoconductive element or scattered around in the event of image transfer deposits on the belt, bringing about the offset of paper sheets or defective image transfer. To solve this problem, cleaning means for removing the toner undesirably deposited on the belt is essential.
Japanese Patent Laid-Open Publication Nos. 7-64444 and 9-152788, for example, each disclose an electric field type of cleaning means using a cleaning member implemented by a conductive member. A cleaning bias opposite in polarity to a charge deposited on toner is applied to the cleaning member. The cleaning member slidingly contacts the surface of the belt, causing the toner to electrostatically move from the belt toward the cleaning member. In this type of cleaning means, an electric field that causes the toner to move toward the conductive member away from the belt (bias electric field hereinafter) is formed between the conductive member and the belt.
The electric field type of cleaning means, however, cannot achieve sufficient performance and sometimes fails to fully clean the belt. This is particularly true when the belt has a volume resistivity of 1012 xcexa9cm to 1013 xcexa9cm or above or when a great amount of charge deposits on the toner.
Specifically, dielectric polarization occurs more easily with a belt having high resistance than with a belt having low resistance. This, coupled with the fact that a belt with high resistance easily retains a charge applied thereto during image transfer and opposite in polarity to the charge of the toner, intensifies a force electrostatically attracting the toner onto the belt. When a great amount of charge deposits on the toner, the attraction electrostatically attracting the toner onto the belt also increases due to a Coulomb""s force or an image force. The toner deposits on the belt in the form of layers. Therefore, if the electrostatic attraction acting between the toner and the belt is intense, even the toner in an upper layer sometimes fail to move toward the conductive member despite the electric field. In light of this, the cleaning bias to be applied to the conductive member may be increased in order to provide the cleaning means with a desirable cleaning ability even in the above condition.
However, an excessive cleaning bias is apt to cause dielectric breakdown to occur in the direction of thickness of the belt in the position where the cleaning member and belt contact each other, resulting in current leakage. As a result, the bias electric field is not formed and makes a sufficient cleaning ability unachievable. Moreover, an excessive cleaning bias is likely to inject a charge opposite in polarity to the toner into the toner existing on the belt and invert the polarity of the toner. The toner inverted in polarity cannot be removed by the electric field type of cleaning means and degrades the cleaning ability. In addition, the toner deposited on the background of the photoconductive element and inverted in polarity is apt to deposit on the belt when the former is brought into contact with the latter, also degrading the cleaning ability.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 6-35340 and 11-38777 and Japanese Patent No. 2,954,812.
It is therefore an object of the present invention to provide an image transferring device capable of cleaning the surface of a belt more efficiently than the conventional electric field type of cleaning means even when the belt has high resistance or when a great amount of charge deposits on toner, while removing even toner inverted in polarity, and an image forming apparatus including the same.
In accordance with the present invention, an image transferring device for transferring a toner image from an image carrier to a recording medium includes a belt passed over a plurality of rotary bodies for supporting and conveying the recording medium and a cleaning member for cleaning the surface of the belt in contact therewith. A dielectric layer forms the surface of the cleaning.
Also, in accordance with the present invention, an image forming apparatus includes an image carrier, a toner image forming device for forming a toner image on the image carrier, and an image transferring device for transferring the toner image from the image carrier to a recording medium. The image transferring device includes a belt passed over a plurality of rotary bodies for supporting and conveying the recording medium and a cleaning member for cleaning the surface of the belt in contact therewith. A dielectric layer forms the surface of the cleaning member.